The present invention relates to an alarm system for detecting changes in load on terrain and more particularly to an alarm system containing a pressure-sensitive switch and a mat containing a flexible element in which a change in volume caused by external action on the mat is detected by the switch. Various alarm systems are known in which an odd event or an event at an odd time on a given part of the terrain triggers an alarm or a display signal.
These systems comprise pressure pick-ups arrayed in a conventional manner, whereby the number of pressure pickups provided determines the effectiveness of the system. The pickups generally act individually in an electrical, mechanical electronic or pneumatic manner on a signal-processing system that carrys out the desire processing of the signals. Such systems are used mostly to protect spaces, buildings, fenced-in lots and fences or gates. Because of the restricted radii of detection of such sensors, any change in their location requires a thorough study of the greatest probability and danger of the expected odd events. Furthermore, an unavoidable drawback is that such alarm systems can be set up only with a predictable probability of success and may be fairly easily circumvented once their array is known.
An effective and reliable alarm system must be such that
(1) it is practically uncircumventable, PA1 (2) it evidences high sensitivity to changes in load under all atmospheric conditions, PA1 (3) it responds only to transiently effective changes in load, not only once, but reversibly, and PA1 (4) gradual changes in environmental factors, for instance local changes in the weight of the soil above or temperature variations, remain ineffective.
These requirements are in part contradictory. Thus, an extension of the sensitive zone also must cause a volume increase in the sensor. The relative change in volume and the change in pressure corresponding thereto, to which the signal processing system will respond, will, of course, be smaller, as the sensitive zone of the sensor becomes larger. Thus, the sensitivity of detection decreases.
One of the most progressive and advanced solutions to date has been suggested in the German Offenlegungsschrift No. 2,040,762 or U.S. Pat. No. 3,719,939. Two flexible tubes filled with an incompressible medium and spaced apart are buried in the terrain to be protected. A converter or transducer, that senses the disturbance acting on a particular medium-filled tube, is connected to each. A distance disturbance or a change in the environment as well as a pressure applied equally to both tubes affects each tube similarly in a two-tube system, resulting in a null signal in a balancing circuit at the transducer output. Only if there is a local disburbance affecting only one of the tubes, does the balancing circuit emit an electrical output signal which is a function of the pressure difference between the two tubes. This signal is available to trigger an alarm indicating unauthorized penetration of the bounded area.
This system meets only a few of the above requirements and suffers from the essential drawback that such sensors offer a reliable barrier only if laid down in high density; this is so because the changes in load due to an intruder are short-range because of the soil bridging the buried tubes and furthermore they become wholly inoperative if this soil freezes.
Even though this detection system acts differentially through a differential-pressure transducer, it is subject to long term or permanent deviations on account of slow, locally varying loads or temperature fluctuations. The signal may be reset electrically, but not mechanically, to the initial position, and this may lead to overloading of the pick-up, at least to decreasing its sensitivity. In any event, the pick-up must be designed for fairly high pressure differences, and this means an inherent loss in sensitivity.